Technology Computer Aided Design (TCAD) uses physics-based computer simulations to design, analyze, and optimize semiconductor devices. TCAD represents the available physical knowledge of semiconductor processing and devices in terms of computer models. It represents devices as one dimensional, two-dimensional or three-dimensional finite-element or finite-volume models. Each element represents a piece of a certain material, with certain properties. TCAD numerically solves partial differential equations in space and/or time with appropriate boundary conditions. Typically this is done with finite element or finite volume analysis, although in some cases other methods for solving the partial differential equations can be used, such as particle/atomistic methods.
TCAD consists of two major components:    1. Process simulation is modeling semiconductor manufacturing processes. The simulation starts with the bare wafer and finishes with device structures. Processes such as implantation, diffusion, etching, growth, and deposition processes are simulated on a microscopic level.    2. Device simulation is modeling the semiconductor device operation on a microscopic level. By integrating microscopic currents, the electrical behavior is characterized. SPICE model parameters can be extracted from the simulated electrical characteristics.
Today's TCAD tools are capable of modeling the entire semiconductor manufacturing process and product performance with physical models of varying sophistication. Typically, from the simulation-derived device characteristics it is possible to extract model parameters for so-called (lumped) circuit models, i.e., which can be used in circuit simulators such as SPICE, which is often used as a central tool in Electronic Computer Aided Design (ECAD) to generate circuits.
While in TCAD typically few semiconductor devices are simulated with very high sophistication in terms of physical models, in ECAD more devices can be simulated, but the models for the individual devices are enormously reduced in complexity and sophistication. TCAD is applicable to all semiconductor devices, notably diodes, transistors, optical devices such as LEDs, lasers, specific test structures for process control, and others.
It is known to use circuit simulations during the manufacturing process of a semiconductor product. E.g. in JP 2001/188816 a method for manufacturing a transistor is described, where the gate length and the gate width are calculated using a circuit model. The calculation is performed on the basis of measurement results during the manufacturing.
TCAD allows an understanding of the manufacturing process and the operation of semiconductor devices and is therefore often used in research and development for the development of new processes and devices. Notably, it allows to save cost by reducing the number of costly experiments.
One goal of the present invention is to use TCAD to address issues of process and device variability in manufacturing. The simulation experiments in TCAD have the advantage that every process condition can be accurately controlled, and that arbitrary product performance characteristics can be determined. This is as opposed to real experiments, where the control of process steps may be difficult and subject to uncontrollable drift or variation in the equipment, and where the limitations of metrology can make it difficult, expensive or impossible to make measurements both in non-destructive and destructive measurements.
In particular, it is important to improve the systematic yield in semiconductor manufacturing of products where the structure of the semiconductor product is smaller than 130 nm. In this range the yield is increasingly subject to other limiting factors than just defects. Notably, addressing the parametric yield issues that arise through process and device variability is very important. Nonetheless, aspects of the present invention can be used advantageously also for structures above 130 nm.
Unfortunately, while TCAD simulations can be made very accurate, they are still very time consuming to execute. A single reasonably accurate 2-dimensional simulation of a MOSFET device may take on the order of an hour or more to execute. This deficiency severely limits the practicality of using TCAD in manufacturing, as opposed to design and development.